[Spice-devel] [PATCH spice-common 1/2] canvas_base: Check for overflows decoding LZ4

[Frediano Ziglio]

> 
> On Tue, 2018-07-10 at 14:35 +0100, Frediano Ziglio wrote:
> > Check we have enough data before reading.
> 
> Check *that* we have...
> 

updated, thanks

> > This could lead to read buffer overflows being undetected.
> > This is not a security issue, read happens only in the client not
> > causing
> > any information leakage, maximum can generate a crash or some garbage
> > on
> > the screen.
> > 
> > Signed-off-by: Frediano Ziglio <fziglio at redhat.com>
> > ---
> >  common/canvas_base.c | 23 +++++++++++++++++++----
> >  1 file changed, 19 insertions(+), 4 deletions(-)
> > 
> > diff --git a/common/canvas_base.c b/common/canvas_base.c
> > index 05c4e1f..ae07625 100644
> > --- a/common/canvas_base.c
> > +++ b/common/canvas_base.c
> > @@ -537,6 +537,9 @@ static pixman_image_t *canvas_get_lz4(CanvasBase
> > *canvas, SpiceImage *image)
> >      width = image->descriptor.width;
> >      stride_encoded = width;
> >      height = image->descriptor.height;
> > +    if (data + 2 > data_end) {
> > +        return NULL;
> 
> All other error conditions that you handle below print a warning. Do
> you want one here as well?
> 
> Acked-by: Jonathon Jongsma <jjongsma at redhat.com>
> 

does

       g_warning("missing header in LZ4 data");

look fine?

Frediano

> 
> > +    }
> >      top_down = !!*(data++);
> >      spice_format = *(data++);
> >      switch (spice_format) {
> > @@ -579,16 +582,22 @@ static pixman_image_t
> > *canvas_get_lz4(CanvasBase *canvas, SpiceImage *image)
> >      bits = dest;
> >  
> >      do {
> > +        if (data + 4 > data_end) {
> > +            goto format_error;
> > +        }
> >          // Read next compressed block
> >          enc_size = read_uint32_be(data);
> >          data += 4;
> > +        /* check overflow. This check is a bit different to avoid
> > +         * possible overflows. From previous check data_end - data
> > cannot overflow.
> > +         * Computing data + enc_size on 32 bit could cause
> > overflows. */
> > +        if (enc_size < 0 || data_end - data < (unsigned int)
> > enc_size) {
> > +            goto format_error;
> > +        }
> >          dec_size = LZ4_decompress_safe_continue(stream, (const char
> > *) data,
> >                                                  (char *) dest,
> > enc_size, available);
> >          if (dec_size <= 0) {
> > -            spice_warning("Error decoding LZ4 block\n");
> > -            pixman_image_unref(surface);
> > -            surface = NULL;
> > -            break;
> > +            goto format_error;
> >          }
> >          dest += dec_size;
> >          available -= dec_size;
> > @@ -599,6 +608,12 @@ static pixman_image_t *canvas_get_lz4(CanvasBase
> > *canvas, SpiceImage *image)
> >  
> >      LZ4_freeStreamDecode(stream);
> >      return surface;
> > +
> > +format_error:
> > +    spice_warning("Error decoding LZ4 block\n");
> > +    LZ4_freeStreamDecode(stream);
> > +    pixman_image_unref(surface);
> > +    return NULL;
> >  }
> >  #endif
> >  
>